Volumetric Efficiency – Enginology

Manipulating Torque Curves

Over time, in this column, we have touched upon several aspects of not only how to achieve higher levels of volumetric efficiency (as it relates to torque) but the importance of this feature in a racing engine, particularly for circle track applications. In the course of those discussions, we’ve attempted to establish an understanding of how induction and exhaust system design and dimensions play into this subject. But before we try to develop topics specific to yet another perspective of how torque curves can be linked with these two major systems, a brief review of a few previously-mentioned points is intended to provide some background information.

For example, we know that in both an intake and exhaust system, we’re dealing with pulsating, unsteady flow. We also know that as a particular function of piston displacement and rpm, each system can pass through what we’ll call a “resonant” point often associated with a peak torque value. At this particular rpm, both systems are generally the most efficient and can be associated with a certain “mean flow velocity” found at such peaks, regardless of engine combination. We can also label this flow rate as the “critical” velocity.

As each system approaches this flow velocity, there is not sufficient rpm to achieve the rate and beyond peak torque rpm it will exceed that value, thus the shape of a typical torque curve. We also know, from a design standpoint, that each can be treated as a separate system. That is to say each will effectively produce its own torque curve. In reality, they combine to create a “net” or resulting curve shape just described.

It turns out that a third variable influencing at what rpm an engine of specific piston displacement will reach its critical intake or exhaust flow velocity is the cross-section area of the flow path. For the sake of simplicity, let’s say these paths are of constant cross-section, neither tapered nor “stepped” as in practice. Actually, for purposes of our discussion, this aspect of the subject is irrelevant. What’s important is understanding that it’s possible to “tune” intake and exhaust systems separately. Let’s restate that suggestion.

On a given engine, it’s possible to select passage section areas to achieve peak torque (volumetric efficiency) points at specific rpm where such boosts are desired. Now, let’s consider the possible value in having this measure of influence over an engine’s torque characteristics.

Suppose we decided to have an intake and exhaust system broaden (or make flatter) a net torque curve. One approach would be to adjust intake and exhaust passages sufficiently different to spread their respective torque peaks farther apart in the rpm range. The net effect of this would be to create somewhat of a “depression” in the curve’s shape between the two peaks and, actually, remove some of the net peak torque.

However, the effect could benefit off-the-corner acceleration during the lower engine speeds while having some torque in reserve (higher rpm peak or boost) getting past the flag stand. We’re not dealing with “maybe so” issues here, there have been numerous instances where, when reduced to practice, the concept works. There are patents verifying the results.

Now let’s take this approach one step further. Building on the same concept that flow passage size, piston displacement, and rpm are closely tied with where in an engine’s speed range torque boosts occur. Here’s another thought; suppose we configured a set of headers with two sizes of primary pipes for a V-8 engine, choosing to order the pipes in a way that paired every other cylinder with the same size pipe. As an example, if we assume a firing order of 1-8-4-3-6-5-7-2, every other cylinder would be served by the same size primary pipe. Functionally, we will have created a header system that treats the engine like two V-4s, each of which will contribute to the overall torque curve at separate and different rpm, but do so with a flatter “exhaust system torque curve,” if you will.

The same approach can be applied to the intake manifold for this same engine. While it may no longer be available from Edelbrock, there was a time several years ago when it produced an early version of its Victor series intake manifold for small-block Chevrolet V-8s that was called the Victor 4X4.

The four inboard runners (Nos. 4, 6, 3, and 5), being shorter in length, were sized larger in section area than the four outboard runners (Nos. 2, 8, 1, and 7). These latter, being longer, were sized with a slightly smaller section area based on a naturally lower tuning rpm because of their comparatively longer length, compared to the shorter inboard runners. By design, the manifold was intended to help broaden or flatten the resulting torque curve and was largely targeted for oval-track engines.

After the manifold’s introduction, I distinctly recall a conversation with Junior Johnson who asked if the same approach could be taken with a comparable manifold for the big-block he was using in NASCAR at the time. The conversation included the idea possibly being applied to a header system. I shared with Junior that I’d also done some study on having a camshaft ground with two sets of intake and exhaust lobes (and position on the shaft) to coincide with intentions from the “modified” intake and exhaust systems, further enhancing the “two V-4s” notion for manipulating the torque curve. Post-discussion results indicated he was successful.

What I hadn’t shared with him was the fact I was driving (at the time) a then-current model year small-block Chevy Camaro using just such a camshaft. In later years, a short-track engine builder with whom Edelbrock was working adapted the idea to a couple of his customer engines with predictably good results. As it has turned out, this approach to “customizing” camshafts became a method to help resolve cylinder-to-cylinder volumetric efficiency variations by tailoring lobe specifications to compensate for imbalances in torque among an engine’s cylinders. That practice continues today.

The overriding point here is that it’s possible to configure intake, exhaust and camshaft packages to put torque at more favorable engine speeds than obtainable by some other means. In fact, the ability to identify areas in a given engine speed range where torque boosts can be helpful becomes a tool for matching overall torque curves to gearing and track conditions.

If you accept that the area under the torque curve represents available “work” to propel the car, it’s possible to decrease what we’ll call peak torque values by shifting torque to rpm where it’s more helpful without creating more or less gross torque. We know of specific examples where it was known in what speed range an engine operated most frequently, and then by the methods we’ve been discussing, torque enhancement was directed to these rpm-not unlike how you might address the same issue with gear combinations but in addition to this approach. Magic it’s not.

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